This invention relates in general to electrostatographic copying and more specifically to an improved method of making finely divided photosensitive trigonal selenium particles.
In the art of electrostatographic copying, a photosensitive plate containing a photoconductive insulating layer is first given a uniform electrostatic charge in order to sensitize its surface. The plate is then exposed to a light/shadow pattern of activating electromagnetic radiation which selectively dissipates the charge in the illuminated areas of the photosensitive plate while leaving behind a latent electrostatic image corresponding to the non-illuminated areas. The latent image may be developed and made visible by depositing finely divided electroscopic marking particles or suitable liquid ink on the surface of the plate. This concept was originally described by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the field.
Conventional xerographic plates or drums usually comprise a photoconductive insulating layer overlaying a conductive support. A photoconductive material which has had wide use as a reusable photoconductor in commercial xerography comprises vitreous or amorphous selenium. Vitreous selenium in essence comprises super cooled selenium liquid and may readily be formed by vacuum evaporation by cooling the liquid or vapor so suddenly that crystals of selenium do not have time to form. Although vitreous selenium has had wide acceptance for commercial use in xerography, its spectral response is limited largely to the blue-green portion of the electromagnetic spectrum below about 5200 Angstrom Units. In general, one requirement of a photoconductor, such as vitreous selenium, is that its resistivity should drop at least several orders of magnitude in the presence of activating radiation or light in comparison to its resistivity in the dark. Also, the photoconductive layer should be able to support a significant electrical potential in the absence of radiation.
Selenium also exists in a crystalline form known as trigonal or hexagonal selenium which is well known to the semiconductor art for use in the manufacture of selenium rectifiers. In the crystalline trigonal form, the morphology or structure of the selenium consists of helical chains of selenium atoms which are parallel to each other along the crystallographic c-axis. Trigonal selenium is not normally used in xerography as a homogeneous photoconductive layer because of its relatively high electrical conductivity in the absence of activating radiation, although in some instances trigonal selenium can be used in binder structures wherein trigonal selenium particles are dispersed in a matrix of another material such as an electrically insulating resin, an electrically active organic material, or a photoconductor such as vitreous selenium. Trigonal selenium has an advantage over amorphous selenium in that, unlike amorphous selenium, it is sensitive to radiation of wavelengths over most of the visible spectrum.
U.S. Pat. Nos. 2,739,079 and 3,692,521 both describe photosensitive members utilizing small amounts of crystalline hexagonal (trigonal) selenium contained in predominantly vitreous selenium matrices. In addition, copending U.S. patent application Ser. No. 669,915, filed Sept. 22, 1967, describes a special form of red hexagonal selenium suitable for use in binder structure in which finely divided red hexagonal selenium particles are contained in a resin binder matrix.
Although trigonal selenium exhibits a wider spectral response than vitreous selenium, as stated above, trigonal selenium is not normally used in xerography because of its relatively high electrical conductivity in the dark. However, imaging structures which are able to use hexagonal selenium particles would have advantages over those using vitreous selenium with regard to improved spectral response. Further, the use of trigonal selenium in xerographic members, especially in the binder form, would provide greater ease in the manufacture of the photoconductive device in that the expensive vacuum coating apparatus required for forming vitreous selenium would not be necessary in forming a binder layer containing trigonal selenium particles. Binder layers are also inherently more flexible than evaporated layers. In addition, solvent coated binder layers can adhere more tenaciously to substrates than conventional vacuum evaporated layers.
While methods are known for precipitating crystalline selenium from an alkali solution of selenide or polyselenide ions by the addition of an oxidizing agent (see for example U.S. Pat. No. 1,915,703 to Towne et al) the prior art methods are not particularly suitable for preparing trigonal selenium particles having the physical and electrical characteristics necessary for effective use in electrostatographic photoreceptors. This is the case because in the absence of careful control over the precipitation of trigonal selenium both in terms of temperature and relative concentration of reactants, the selenium precipitated is unsuitable for the use contemplated. This is because failure to properly control the precipitation may result in large rod-like crystals being precipitated. These crystals are undesirable because it is difficult to reproducibly fabricate a uniform electrostatographic binder layer photoreceptor from them. In addition, the failure to carefully control both synthesis and post synthesis parameters can lead to forms of trigonal selenium which lack acceptable electrical properties.
It would be desirable, and it is an object of the present invention to provide a novel method for the preparation of trigonal selenium.
Another object is to provide such a method which produces submicron, generally spherical particles of trigonal selenium.
An additional object is to provide a method for the preparation of trigonal selenium particles which exhibit good photoconductivity and acceptable resistivity in the dark.